Stable telomere length and telomerase expression in differentiation from naive to memory B lymphocytes: Telomere length and telomerase activity play important roles in regulating replicative lifespan of cells. The length of telomeres also serves as a marker for the replicative history and for the remaining replicative potential of cells. Differential telomere length has been reported in human naive and memory T cells but not in naive versus memory B-lymphocytes. Recently, we have analyzed telomere length, telomerase expression, and ratio of peripheral blood naive and memory B cells with age. The dynamics of telomere loss and telomerase activation are quite similar between naive and memory B cells with age. Although there is an accumulation of memory B cells with age, there appears to be a quantitative difference between B and T cells. Remarkably, we observed that there is no obvious loss of telomere length during differentiation from naive to memory B cells in all age groups. Furthermore, both naive and memory B cells are capable of inducing telomerase activity at similar levels after in vitro stimulation independent of donor's age. Finally, there is a slow increase of memory B cells in peripheral blood with age. Together, these findings suggest that B cells are capable of maintaining telomere length during differentiation from naive to memory B cells and this ability is maintained through age. These unexpected findings demonstrate that T and B lymphocytes, although closely related in immune function and arising from common hematopoietic progenitor cells, nevertheless employ quantitatively or qualitatively different mechanisms for regulation of telomere length and homeostasis. Further studies are warranted to determine the physiological significances of distinct regulation of telomere length between T and B cells and of the relative stable level of naive/memory B cell in peripheral blood with age.